Digitizers provide values in a given coordinate system that define the location of a pointing device on the digitizer tablet's surface. The operator typically manipulates the pointing device over the tablet's surface using a probe, for example, for tracing a pattern or design, or for creating a pattern or design. Typically, the tablet is connected to a host computer which displays on its monitor the traced or created pattern or design. The probe is commonly a stylus or a cursor that is either capacitively or inductively coupled to the tablet's surface. In many digitizers, the same controlling software and signal processing circuits are used for determining both X and Y coordinates of the pointing device position. In such digitizers, the signals coupled between the pointing device and the tablet's surface are sampled sequentially--say, first for the X coordinate, and then next for the Y coordinate--so that the respective signals for each of the coordinates are received and processed at different times by the digitizer. The signal processing occurs in real time, so that the user can observe displayed on his monitor the design or pattern while it is being traced or created.
One problem frequently encountered with digitizers that use sequential sampling of the coordinate value signals is their inaccurate responsiveness to changes in the location of the pointing device on the tablet surface. Depending upon the speed of motion of the pointing device, one location coordinate provided by the digitizer will lag behind the other location coordinate, sometimes referred to as "hysteresis". This hysteresis problem in the sampling of location signals is particularly annoying in tracing and free-hand drawing applications in which the continuous stream of data produced by the digitizer is recorded in memory as distinct from point-by-point operations in which only the coordinates of selected individual points are recorded.
Another hysteresis-related problem encountered when using digitizers that sequentially sample coordinate values is their sensitivity to relative motion of the probe especially when moved over a curved path. The reported probe positions do not accurately reflect the positions actually occupied by the probe while following this curved path.
These relative motion distortions can be reduced by algorithms that smooth the values provided by the digitizer apparatus. For example, U.S. Pat. No. 4,686,331 discloses the use of a smoothing algorithm for compensating the error in sequentially sampled coordinate values. However, this algorithm is retrospective, i.e., the delays associated with this calculated smoothing of the signal values adds to the delay inherent in the task of calculating coordinate values from the location signals that are sensed sequentially. Such retrospective smoothing does not improve overall digitizer performance, in that it compounds the problem of lack of responsiveness that is characteristic of digitizers using sequential sampling.
Another possible solution is described in U.S. Pat. No. 4,255,617, which is based on calculating the pointing device velocity in the direction of the first coordinate axis measurements made, and then reporting an adjusted first coordinate value based on where the pointing device should be when the second coordinate is measured. This suffers from similar problems to that described above because it assumes a constant velocity for the pointing device. This is not often the case during tracing and free-hand tablet uses when the probe is more likely to follow a curved path, in which the velocity along a coordinate is not constant.